1. Field of the Invention
The present invention relates to an acoustic transducer and to an image generation apparatus. More particularly, the present invention relates to an acoustic transducer which transmits an acoustic primary wave and receives an acoustic secondary wave and to an image generation apparatus using a plurality of the acoustic transducers.
2. Description of the Related Art
It has been known that when an acoustic wave of a large amplitude is radiated, a waveform distortion (a change in waveform) occurs due to the nonlinearity of a propagation medium such that the waveform of the propagated wave differs from the radiated original waveform (initial waveform).
The phenomenon of the occurrence of this distortion appears particularly frequently in an ultrasound region exceeding audible frequencies. Moreover, it is known that waveform distortion does not occur abruptly at a certain distance from the acoustic transducer but appears in a wide ultrasound frequency range based on a distortion accumulation effect such that the amount of distortion increases gradually with propagation. The radiated acoustic wave of the initial waveform is hereinafter referred to as “acoustic primary wave”. Also, a nonlinear acoustic wave generated based on the above-described distortion accumulation effect is referred to as “acoustic secondary wave”.
When an acoustic primary wave is formed of a single frequency component, acoustic secondary waves having integer-multiple frequency components are generated. These waves are called a second harmonic signal, a third harmonic signal, an nth harmonic signal and so on. Medical ultrasound imaging apparatuses using these waves are on the market.
On the other hand, when an acoustic primary wave is formed of two frequency components, a sum signal and a difference signal are generated at two frequencies as nonlinear acoustic waves. The latter is ordinarily called a parametric signal. In some case, each of these signals is called a parametric signal. The parametric signal is an acoustic secondary wave of a frequency |f1−f2| or |f1+f2| generated by a nonlinear effect of an acoustic primary wave having a frequency of f1 and an acoustic primary wave having a frequency of f2 as the acoustic primary waves propagate. The parametric signal has such a characteristic that its amplitude increases by the accumulation effect with propagation. Description will be made by assuming that parametric signals referred to below are difference signals unless otherwise noted.
The parametric signal has such a characteristic as to be capable of setting a long propagation distance (i.e., a large penetration depth) because its frequency is lower than that of the acoustic primary wave. Further, the parametric signal is known to have, as its other characteristics, in comparison with a fundamental wave of the same frequency, a reduced amount of expansion of the beam (i.e., high lateral resolution) and freedom from forming a sidelobe (i.e., having high contrast resolution).
On the other hand, the parametric signal also has a characteristic of being inferior in resolution in the depth direction than the acoustic primary wave (fundamental wave) because its frequency is lower than that of the acoustic primary wave. Further, in parametric imaging using the parametric signal, it is preferable to suppress the acoustic primary wave component which mixes with the parametric signal and harmonics of the parametric signal to lowest possible levels. The measures taken to do so enable high S/N ratio imaging. This condition is the same as the necessity of a pulse inversion technique in harmonic imaging already put to practical use.
In the pulse inversion technique, a voltage signal in a normal phase having a component of a frequency f is first applied to transmit an acoustic primary wave in the normal phase and, after a time interval from this transmission, a voltage signal in the opposite phase having a component of the frequency f is applied to transmit an acoustic primary wave in the opposite phase. The fundamental wave components of these two acoustic waves are wholly identical in waveform to each other, though phase-inverted relative to each other. Thus, ultrasounds in phase opposition to each other are transmitted at different times.
However, harmonics, e.g., the second harmonics are squared to form positive acoustic secondary waves irrespective of whether the polarity of the applied voltage is positive or negative. When two groups of acoustic primary wave components and acoustic secondary wave components such as described above are added together, only acoustic secondary waves, which are harmonic components, remain, while the acoustic primary components become zero when added together because they are in phase opposition to each other.
As a method of adding together two acoustic waves, a method is known in which a signal obtained by converting a preceding received acoustic wave into voltages is temporarily stored in a memory and is read out when a signal obtained by converting a following received acoustic wave into voltages is received, and addition processing is performed on the signals to extract only harmonic components.
It is well known that remaining of the fundamental wave component relates or contributes to the generation of a sidelobe and can be a cause of a speckle. Also, the amount of speckle is increased with increase in the amount of the fundamental component remaining.
As described above, the conventional pulse inversion technique is a method to suppress a fundamental wave component by canceling out acoustic primary waves inverted on the time axis. As a proposition relating to the method to suppress in this way an acoustic primary wave which is a fundamental wave component, a proposition has been made in the field of audible sound parametric speaker techniques to solve a problem that an audible sound reproduced by a parametric speaker contains large amounts of harmonic distortion and intermodulation distortion, is inferior in sound clarity to a corresponding sound reproduced by an electrodynamic speaker and, in some case, causes a listener to have a feeling of discomfort. A technique according to this proposition is disclosed, for example, in a document: Tomoo Kamakura, Shinichi Sakai, Hideyuki Nomura, Masahiko Akiyama “Parametric audible sounds by phase-cancellation excitation of primary waves.”, J. Acoust. Soc. Am. Volume 123, Issue 5, pp. 3694-3694 (May 2008).
This proposition is a method which enables suppression of acoustic primary waves and generation of acoustic secondary waves in an audible sound region by transmitting acoustic primary waves in phase opposition to each other and by utilizing a sound field region where acoustic primary waves can be suppressed.
Propositions about a case of ultrasound in which a parametric acoustic wave is propagated in a living tissue or water have also been made.
For example, an ultrasound image processing method using a parametric acoustic transducer method has been proposed in which an amplitude-modulated wave produced by amplitude modulation on a center frequency through a certain frequency width or an ultrasound having two frequency components is transmitted from an ultrasound probe to a subject; an echo generated in the subject and containing at least a differential frequency component is received by the ultrasound probe; and the received echo is signal processed to obtain an ultrasound image. A technique according to this proposition is disclosed, for example, in Japanese Patent Application Laid-Open Publication No. 8-80300. According to this proposition, diagnosis by means of an ultrasound image with a small attenuation is enabled.
A method has also been proposed in which an ultrasound having a first frequency component and an ultrasound having a second frequency component are transmitted from a first ultrasound transducer and a second ultrasound transducer, respectively, at a desired angle from each other so as to intersect each other at a target diseased part, and a difference signal, a sum signal, a harmonic signal and a frequency-divided signal contained in a reflection signal from the diseased part position at which the waves intersect each other are received by the first ultrasound transducer. A technique according to this proposition is disclosed, for example, in Japanese Patent Application Laid-Open Publication No. 2003-116848. The parametric diagnostic apparatus according to this proposition is capable of clearly displaying the shape of a diseased part or the like.
In each of the techniques according to the above-described propositions, the characteristics of a parametric signal, i.e., a small attenuation, capability of propagating an acoustic wave signal through a large distance, and a phenomenon in which the directionality is higher than that of a fundamental wave of the same frequency, are utilized.